1. Field of the Invention
The present invention relates to an apparatus for preventing malfunction of an electric shutter device when a drop of source voltage occurs.
2. Description of the Prior Art
In an electric shutter device in which a magnet is used to finally transmit an operation from the electric circuit part to the mechanical part, it is known to bring the operation of the magnet into relation with the source voltage and to automatically determine whether photographing is possible or not depending upon whether the operation of the magnet is proper or not. Such a technique is disclosed, for example, in Japanese Patent Application Laid Open No. 16419/1974 the counterpart of which is U.S. Pat. No. 3,875,580 and Japanese Patent Application Laid Open No. 31322/1974 the counterpart of which is U.S. Pat. No. 3,829,873.
According to one embodiment of the known technique there is provided an armature having a certain preset reaction force against the attraction force of the magnet. When the source voltage drops somehow, the magnet fails to attract the armature because of the reduced attraction force of the magnet which can no longer overcome the preset reaction force of the armature. This failure in attraction is detected as an improper action of the magnet. The detection signal is used to prevent opening or closing of the shutter device. In other words, the prior art apparatus works with an electromagnet whose attraction level is directly related with the level of the source voltage. This system of preventing malfunction of an electric shutter device according to the prior art has some disadvantages and is poor in reliability.
For example, if the space between the magnet and armature is changed by dust occasionally put on the armature, then the attraction force of the magnet acting on the armature also may be changed. In this case, the normal value of source voltage predetermined as a threshold level for the aimed purpose can no longer function as designed. Another trouble takes place also in such case where the magnet is attracting the armature with the source voltage being at a level very close to the normal value. Since, in this case, the attraction force acting on the armature is very small, the armature may be moved away from the magnet by only a slightest impact accidentally applied thereto. As easily seen from these examples only, the malfunction preventing apparatus according to the prior art lacks reliability and needs improvements.